Node
A point along the stem of a plant at which leaves are attached.
Nodes of Ranvier
The small gaps in the myelin sheath between successive glial cells along the axon of a
neuron; also, the site of high concentration of voltage-gated ion channels.
Nomograph
A graph that allows a third variable to be measured when the values of two related variables
are known.
Noncompetitive inhibitor
A substance that reduces the activity of an enzyme by binding to a location remote from the
active site, changing its conformation so that it no longer binds to the substrate.
Noncyclic electron flow
A route of electron flow during the light reactions of photosynthesis that involves both
photosystems and produces ATP, NADPH, and oxygen; the net electron flow is from water
to NADP+.
Noncyclic photophosphorylation
The production of ATP by noncyclic electron flow.
Nondisjunction
An accident of meiosis or mitosis, in which both members of a pair of homologous
chromosomes or both sister chromatids fail to move apart properly.
Nonpolar covalent bond
A type of covalent bond in which electrons are shared equally between two atoms of similar
electronegativity.
Nonprotoplasmic
The complex, semifluid, translucent substance that constitutes the living matter of plant and
animal cells and manifests the essential life functions of a cell. Composed of proteins, fats,
and other molecules suspended in water, it includes the nucleus and cytoplasm.
Nonsense mutation
A mutation that changes an amino acid codon to one of the three stop codons, resulting in a
shorter and usually nonfunctional protein.
Non-sister chromatids
chromatid from two different homolog chromosomes.
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